Doctors say the girl, now 2, has only trace amounts of HIV in her bloodstream and has been able to keep the virus in check without medication. The next step is to try to replicate the results.

The U.S. Centers for Disease Control and Prevention estimates that fewer than 200 babies in the nation were infected with HIV at birth in 2010.

But in developing regions — including sub-Saharan Africa, the site of two-thirds of the world's HIV infections — it's a different story. Every year, 300,000 to 400,000 babies are born infected with HIV, Fauci said.

More than half of those children die within the first year, said UCLA's Nielsen, who conducts research on HIV infection in infants and children. If the triple-drug treatment is proved in clinical trials, the therapy could be a boon for these babies.

Fauci suggested that in addition to such trials, researchers might want to take another look at children who have been on antiretroviral therapies since shortly after birth to see whether any of them had the disease cleared from their bodies.

"You don't want to recommend stopping therapy, but you do want to go back and look very carefully," he said. "It may be that we cured them and we don't realize it."

The Mississippi girl is the second patient in the world known to be functionally cured of HIV. The first was Timothy Brown — better known as the Berlin patient — who had not only HIV but also acute myeloid leukemia. When oncologists gave him a bone marrow transplant in 2007, they selected a donor who had a rare genetic mutation that blocks the HIV virus from entering cells. As a result, Brown is now immune to the virus and remains HIV-free without taking any medication.